Most recently, chemical-mechanical polishing (CMP) processes have been used to planarize the surface of wafers and the like in preparation for device fabrication, or to re-planarize the surface of a partially fabricated wafer so as to aid in the remaining construction steps. The CMP process usually involves holding a thin, flat semiconductor wafer against a rotating polishing pad which is wetted with a slurry, and which is pressed against the polishing pad with a controlled downward pressure. The polishing slurry usually comprises alumina or silica particles in a colloidal liquid suspension, usually water based, and may have chemical etchants added thereto. For example, many water based slurries comprise metal hydroxide compounds (bases) or inorganic acids as etchants, which make the slurry basic or acidic, respectively. A rotating polishing head or wafer carrier is typically used to hold the wafer under a controlled pressure (i.e., force) against the rotating pad, which in turn is supported by a platen. The polishing pad, which is typically adhered to the platen, typically comprises a urethane or polyurethane material.
The chemical-mechanical polishing process usually employs two types of etching processes, a mechanical etching caused by the abrasion of the silica or alumina particles, and a chemical etching caused by the etchants added to the slurry. A common approach is to add a chemical which converts the material to be etched to another form, which may then be more readily removed by the mechanical abrasion of the silica and alumina particles.
The primary focus and use of chemical-mechanical polishing in the art has been as a tool for planarizing surfaces. Typically, a thick layer of material is formed over the irregular wafer surface to be planarized, and then the thick material layer is polished back to the point where the tops of the irregular features on the wafer surface begin to be exposed, at which point the polishing process is stopped. This thick planarizing layer is so thick that its top surface is relatively flat, thereby retaining little, if any, of the irregular bumps of the underlying irregular features. The flatness in the top surface of the planarizing layer enables the CMP process to uniformly polish back the layer to the tops of the irregular features.
Against this backdrop, the inventors contemplate a very different use of the CMP process.